An office machine such as a multifunction peripheral and an image scanning apparatus becomes an essential electronic device in the office. Generally, for continuously feeding a stack of documents to increase the scanning speed, the multifunction peripheral or the image scanning apparatus is usually equipped with an automatic document feeder.
FIG. 1A is a schematic perspective view illustrating a conventional automatic document feeder, which is disclosed in for example Japanese Patent Publication No. 2008-201513. As shown in FIG. 1A, the automatic document feeder 1 comprises a pick-up roller assembly 11, a paper detecting mechanism 12 and a feed roller assembly 13. The paper detecting mechanism 12 comprises a first sensing arm 12a, a second sensing arm 12b, a transverse rod 12c and a sensing switch 12d. The sensing switch 12d is connected with the transverse rod 12c through a connecting lever 12e. 
Please refer to FIG. 1B, which illustrates the condition when the second sensing arm 12b is triggered by a skewed paper. During a scanning process, the paper 14 to be scanned is transported through the paper detecting mechanism 12 by the pick-up roller assembly 11. Then, the paper 14 is clamped by the feed roller assembly 13, and the paper is fed into the internal portion of the automatic document feeder 1. As shown in FIG. 1B, during the paper 14 is fed into the internal portion of the automatic document feeder 1, the paper 14 is skewed. After the front edge 14a of the paper 14 is transported through the pick-up roller assembly 11, a first end 14b of the paper 14 is firstly transported through the second sensing arm 12b, so that a second terminal 12cb of the transverse rod 12c is pressed by the second sensing arm 12b. Meanwhile, the sensing switch 12d is not triggered, the feed roller assembly 13 is not enabled, and the paper 14 is continuously transported by the pick-up roller assembly 11.
Please refer to FIG. 1C, which illustrates the condition when the first sensing arm 12a and the second sensing arm 12b are triggered by the skewed paper 14. As the skewed paper 14 is continuously advanced, a second end 14c of the paper is transported through the first sensing arm 12a. Consequently, a first terminal 12ca of the transverse rod 12c is pressed by the first sensing arm 12a, thereby triggering the sensing switch 12d. After the sensing switch 12d is triggered, the feed roller assembly 13 is still not enabled. In this situation, a skew correcting operation can be performed before the skewed paper 14 is fed into the internal portion of the automatic document feeder 1. During the skew correcting operation is performed, the paper 14 is continuously transported by the pick-up roller assembly 11. As the paper 14 is moved forwardly, the first end 14b of the paper 14 will be contacted with the feed roller assembly 13 and stopped by the feed roller assembly 13 from being continuously advanced. Then, the second end 14c of the paper 14 is continuously moved toward the feed roller assembly 13, so that the front edge 14a of the paper 14 is gradually parallel with the feed roller assembly 13. Meanwhile, the purpose of correcting the skew of the paper is achieved. After the paper 14 is moved through a constant distance, the feed roller assembly 13 is enabled to feed the paper 14 into the internal portion of the automatic document feeder 1 in order to perform the scanning operation.
From the above discussion, it is noted that the automatic document feeder 1 uses the paper detecting mechanism 12 to correct the paper skew when the paper is transported through the paper detecting mechanism 12. After the paper skew is corrected, the sequent scanning operation will be operated more smoothly.
As previously described, by realizing whether the sensing switch 12d is triggered or not, the paper detecting mechanism 12 may determine the timing of enabling the feed roller assembly 13 and the pick-up roller assembly 11 so as to perform the skew correcting operation. The conventional automatic document feeder 1, however, still has some drawbacks. For example, only after the sensing switch 12d is triggered, the conventional automatic document feeder 1 can judge that two ends of the paper 14 are both transported through the paper detecting mechanism 12. However, the skew angle of the paper 14 fails to be accurately realized when the paper 14 is skewed. If the skew angle of the paper 14 is too large, the distance difference of the first end 14b and the second end 14c of the paper 14 with respect to the feed roller assembly 13 is greater than the constant distance. Under this circumstance, even if the paper 14 is fed into the internal portion of the automatic document feeder 1 after the skew correcting operation, the scanned image is possibly distorted or the paper 14 is possibly jammed in the feeding channel. On the other hand, if the skew angle of the paper 14 is too small or even if the paper 14 is not skewed, the skew correcting operation of the paper 14 is still performed by the disabled feed roller assembly 13 during the paper 14 is transported through the paper detecting mechanism 12; and then the paper 14 is fed into internal portion of the automatic document feeder 1 by the feed roller assembly 13. Under this circumstance, the operating time is prolonged, and the overall working efficiency is impaired.
Therefore, there is a need of providing an improved automatic document feeder so as to obviate the drawbacks encountered from the prior art.